Solar cells are typically manufactured using the same processes used for other semiconductor devices, often using silicon as the substrate material. A semiconductor solar cell is a device having an in-built electric field that separates the charge carriers generated through the absorption of photons in the semiconductor material. This electric-field is typically created through the formation of a p-n junction (diode) which is created by differential doping of the semiconductor material. Doping a part of the semiconductor substrate (e.g. surface region) with impurities of opposite polarity forms a p-n junction that may be used as a photovoltaic device converting light into electricity.
FIG. 1 shows a cross section of a representative solar cell 100. Photons 101 enter the solar cell 100 through the top surface 105, as signified by the arrows. These photons pass through an anti-reflective coating 110, designed to maximize the number of photons that penetrate the solar cell 100 and minimize those that are reflected away from the solar cell 100.
Internally, the solar cell 100 is formed so as to have a p-n junction 120. This junction is shown as being substantially parallel to the top surface 105 of the solar cell 100 although there are other implementations where the junction may not be parallel to the surface. The solar cell is fabricated such that the photons enter the substrate through the n-doped region, also known as the emitter 130. While this disclosure describes p-type bases and n-type emitters, n-type bases and p-type emitters can also be used to produce solar cells and are within the scope of the disclosure. The photons with sufficient energy (above the bandgap of the semiconductor) are able to promote an electron within the semiconductor material's valence band to the conduction band. Associated with this free electron is a corresponding positively charged hole in the valence band. In order to generate a photocurrent that can drive an external load, these electron hole (e-h) pairs need to be separated. This is done through the built-in electric field at the p-n junction. Thus any e-h pairs that are generated in the depletion region of the p-n junction get separated, as are any other minority carriers that diffuse to the depletion region of the device. Since a majority of the incident photons are absorbed in near surface regions of the device, the minority carriers generated in the emitter need to diffuse across the depth of the emitter 130 to reach the depletion region and get swept across to the other side. Thus to maximize the collection of photo-generated current and minimize the chances of carrier recombination in the emitter 130, it is preferable to have the emitter 130 be very shallow.
Some photons 101 pass through the emitter region 130 and enter the base 140. These photons 101 can then excite electrons within the base 140, which are free to move into the emitter 130, while the associated holes remain in the base 140. As a result of the charge separation caused by the presence of this p-n junction 120, the extra carriers (electrons and holes) generated by the photons 101 can then be used to drive an external load to complete the circuit.
By externally connecting the emitter 130 to the base 140 through an external load, it is possible to conduct current and therefore provide power. To achieve this, contacts 150a and 150b, typically metallic, are placed on the outer surface of the emitter 130 and the base 140. Since the base 140 does not receive the photons 101 directly, typically its contact 150b is placed along the entire outer surface of the base 140. In contrast, the outer surface of the emitter 130 receives photons 101 and therefore cannot be completely covered with contacts 150a. However, if the electrons have to travel great distances to the contact, the series resistance of the cell increases, which lowers the power output. In an attempt to balance these two considerations; the distance that the free electrons must travel to the contact 150a or 150b, and the amount of exposed emitter surface 160 illustrated in FIG. 2; most applications use contacts 150a that are in the form of fingers. FIG. 2 shows a top view of the solar cell of FIG. 1. The contacts 150a are typically formed so as to be relatively thin, while extending the width of the solar cell 100. In this way, free electrons need not travel great distances, but much of the outer surface of the emitter is exposed to the photons. Typical contacts 150a on the front side of the substrate are 0.1 mm wide, with an accuracy of approximately +/−0.1 mm. These contacts 150a are typically spaced between 1-5 mm apart from one another. While these dimensions are typical, other dimensions are possible and contemplated herein.
A further enhancement to solar cells is the addition of heavily doped substrate contact regions. FIG. 3 shows a cross section of this enhanced solar cell. The solar cell 100 is as described above in connection with FIG. 1, but includes heavily n-doped contact regions 170. These heavily doped contact regions 170 correspond to the areas where the contacts 150a will be affixed to the solar cell 100. The introduction of these heavily doped contact regions 170 allows much better contact between the solar cell 100 and the contacts 150a and significantly lowers the series resistance of the solar cell 100. This pattern of including heavily doped regions on the surface of the substrate is commonly referred to as selective emitter (SE) design. These heavily doped regions may be created by implanting ions in these regions. Thus, the terms “implanted region” and “doped region” may be used interchangeably throughout this disclosure.
A selective emitter design for a solar cell also has the advantage of higher efficiency cells due to reduced minority carrier losses through recombination due to lower dopant/impurity dose in the exposed regions of the emitter layer. The higher doping under the contact regions provides a field that collects the majority carriers generated in the emitter and repels the excess minority carriers back toward the p-n junction.
Such structures are typically made using traditional lithography (or hard masks) and thermal diffusion. An alternative is to use implantation in conjunction with a traditional lithographic mask, which can then be removed easily before dopant activation. Yet another alternative is to use a shadow mask or stencil mask in the implanter to define the highly doped areas for the contacts. All of these techniques utilize a fixed masking layer (either directly on the substrate or upstream in the beamline).
All of these alternatives have drawbacks. For example, the processes enumerated above all contain multiple process steps. This causes the cost of the manufacturing process to be prohibitive and may increase substrate breakage rates. These options also suffer from the limitations associated with the special handling of solar cells, such as aligning the mask with the substrate and the cross contamination with materials that are dispersed from the mask during ion implantation.
Consequently, efforts have been made to reduce the cost and effort required to dope a pattern onto a substrate. While some of these efforts may be successful in reducing cost and processing time, often these modifications come at the price of reduced accuracy. Typically, in semiconductor processes, masks are very accurately aligned. Subsequent process steps rely on this accuracy. For example, referring to FIG. 4, after the heavily doped regions 170a-c have been implanted, contacts 150a are pasted to the substrate. Each of these processes is usually performed relative to some reference mark or fiducial. This mark may be an edge or corner of the substrate, or a specific mark or feature on the substrate. Since each of these process steps is typically referenced to a specific point, it is imperative that a high degree of accuracy be maintained. These efforts to reduce cost and processing steps degrade this accuracy, thereby potentially impacting the performance and yields of the devices made using these methods.
Therefore, there exists a need to produce solar cells where the number and complexity of the process steps is reduced, while maintaining adequate accuracy so that subsequent process steps are correctly positioned. While applicable to solar cells, the techniques described herein are applicable to other doping applications.